Frequency modulated wave detector



June 16, 1942.

w. VAN a. ROBERTS 2,286,378'

FREQUENCY MODULATED WAVE DETECTOR Filed Aug. 31, 1940 wggw A ORNEY vPatented June 16, 19 42 FREQUENCY MODULATED WAVE nn'rnc'roa Walter vanB. Roberts, Princeton, N. J., assignor to Radio Corporation of America,a corporation of Delaware Application August 31, 1940, Serial No.354,983

11 Claims. (01. 250-27) My present invention relates to frequencymodulated wave (FM) detectors, and more particularlyg to novel andimproved FM detectors of the balanced type. i

The main object of the present invention is to provide an FM detectoroperating on the principle of balanced detectors each actuated .solelyby the voltage developed in one of a pair of circuits which are tunedrespectively above andbelow the mean frequency ofthe applied FM signal,and, in particular, to provide a circuit of the above nature which issuitable for quantity manufacture and readily adjusted by a Serviceman.The novel features which I believe to be characteristic of my inventionare set forth in particularity in the appended claims; the inventionitself, however, as to both its organization and method of operationwill best be understood by.

reference to the following description taken in connection with thedrawing in which I have indicated diagrammatically several circuitorganizations whereby my invention may be carried into effect.

In the drawing: 4

Fig. 1 shows a circuit diagram of an FM detector constructed inaccordance with the invention,

Fig. 2 illustrates the detection of the FM detector of Fig. 1,

Fig. 3 is. a graphic analysis of the detection circuit,

Fig. 4 shows various response characteristics of the detector circuit,

Fig. Sis a-circuit diagram of apr'eferred detector embodiment.

-Referring now to theaccompanying drawing, the invention will be.explained in connection with the simplified circuit of Fig. 1. Here thedecharacteristic tectors D1 and D, which may be diodes, are

shunted in reveisedrelation across the .con-

The 'principleof operation may be explaine in part by reference to Fig?1 which shows the two series resonant circuits equally detuned one aboveand the other below the main frequency of the applied FM signal. -Aseparate detector is energized by the voltage across the condenser ineach circuit, although it is also possible to energize the detectorsfrom the voltage developed across the inductive portion of each circuit.When a constant voltage of varying frequency is applied to the parallelcombination of series tuned circuits of Fig. l, the voltage across thedetector in either circuit will vary with frequency, and will have amaximum value which is approximately Q times the'applied voltage. Thesymbol Q is the ratio of coil reactance to coil resistance at theresonant frequency of the circuit.

Curve lof Fig. 2 shows how the voltage on frequency of the applied FMsignals. Curve 3 represents the difference between the magnitudes of thedetector output voltages, or, in other words, the variation of :directcurrent output I from the detection system plotted against frequency.The-foregoing analysis was based on the simplifying assumption of,asource of constant voltage signals, whereas ,no practical vacuum tubeacts as a constant voltage source. In accordance with my presentinvention, however, a vacuum tube amplifier adapted to supply .1

. more nearly constant current than constant voltage may be utilized,nevertheless, to maintain a substantially constant voltage across the"network of Fig. 1.

densers C1 and C2 respectively of the series tuned branches Ll-Cl andL2C2. The FM signals, after conversion to a suitable intermediatefrequency, are shown applied between the junction of Ll-L2 and ground.The low potential terminals of C1 and C2 are at ground potential,

while the properly by-passed resistors R1 and. R: are connected betweenthe diodes D1 and Dz' respectively and ground. Resistors R1 and Ra arethe detector load resistors. The detected output voltage is tapped offfrom a resistor R: which connects the ungrounded ends of the resistorsR1 and R2. The symbols 11 and r: represent the series resistances of'the respective series resonant circuits.

- admittance of a series combination of inductance,

This will now be explained by reference to Fig. 3.

In Fig. 3 the circle represents the locus of the resistance,'andcapacity plotted in the complex plane as frequency varies from the zeroto inflnity. As' frequency increases, starting from zero, the pointrepresenting the admittance of the series circuit starts from the origin0, and travels clockwise around the circle. .Point T- clockwise back tothe'orisin. Thepoint at the top of the circuit is the admittance whenthe frequency is just enough lower than the resonant frequency so thatthe circuit reactance is equal to its resistance. the circle is theadmittance when the frequency is Just enough higher than the resonantfrequency so that the reactance is again equal to the resistance. If theresistances of the two circuits of Fig. 1 are equal the admittances ofthese circuits are both representable by the same circle, and theonlydifference is that due to one of the circuits being tuned above the meanfrequency of the signal and the other one below the mean frequency. Thetwo points representing, respectively the admittances of the twocircuits, will thus be, at any given frequency, at different positionson the circle.

To simplify the discussion let it be assumed that at the mean signalfrequency the admittance of one circuit is represented by Y1, and thatof the second circuit by Y2. The total admittance of the two circuits inparallel is, therefore, the vector sum of the admittances represented bypoints Y1 and Y2. When these points are located as shown, their sum isobviously equal to the conductance represented by point T. If, now, thesignal frequency increases until the first circuit is resonant, thenpoint Y1 moves clockwise on the circle to position T. Coincidentally,point Y2 moves clockwise toward the origin, and comes to a positionwhich represents a relatively small admittance which is, furthermore,preponderantly a susceptance. Hence, when this The point at the bottomof Hence, if we left fm be the mean frequency, we

r r 2f,, 2f. approximately, since 41rfdL has been assumed equal to 1'.Since 2fd is approximately the maximum range of frequencies that can beaccommodated by the useful part of curve 3 of Fig. 2, and since thisrange is about equal to the nominal width of the FM channel, the optimumvalue of the Q ofthe coils of Fig. 1 is roughsmall admittance is addedvectorially to the relatively large conductive admittance represented bypoint T, the resultant is very little greater in magnitude than theconductance of the first circuit alone. Similarly, if the frequency islowered toward the resonant frequency of the second circuit, points Yand Y2 move counter-clockwise with similar results as far as the totaladmittanc of the circuit is concerned.

Thus, the total admittance of the circuit varies with frequencyapproximately as shown in curve a of Fig. 4. This curve is substantiallyconstant in value over the range of frequencies between the resonantfrequencies of the two circuits. If, however, the Q of the circuits hadbeen so related to the difference between their resonant frequenciesthat at the mid-frequency the points Y1 and Y: had each fallen,considerably nearer the origin, then the plot of admittance againstfrequency would have had a pronounced double hump as shown in curve b ofFig. 4. This represents the case where the, Q of the circuit is chosentoo large for the desired difference between the two resonantfrequencies. On the other hand, if the Q is chosen so small that at themid-frequency the points Y1 and Y2 each lie considerably closer to pointT than shown in Fig, 3, then the admittance characteristic has no humps,but is as shown in curve 0 of Fig. 4. It is not desirable to use toohigh a Q value, because, as will be evident later, a fairly constantadmittance over the operating range of frequencies is required. Nor .yettoo low a valve should be used as the latter impairs the steepness andshape of the resultant curve 3 of Fig. 2. The optimum Q value of thecircuits is, therefore, in the vicinity of that value which will makethe resistance of the circuit equal to its reactance at.

is is the difference between the mean frequency and the resonantfrequency of v the' circuit.

ly equal to the ratio of meanfrequency to channel width. Thus, in casefrequency modulation employing a 200 kilocycle (kc.) channel is used,and'assuming a mean frequency of 13 mc., for example, the Q valueshould, therefore, be of the order of 65. It has been shown that asuitable choice of the Q of the circuits of Fig. l will result in asubstantially constant impedance for the parallel combination of thesecircuits. Thus, a constant current supplied to this combination willresult in a substantially constant voltage so that the operation of thecircuits will be as em plained and illustrated by Fig. 2.

The amount of current available from an amplifier tube being relativelysmall, it is a further object of this invention to provide a currenttransforming network for greatly multiplying the current delivered bythe amplifier tube. This is shown more specifically in Fig. 5,'whereinthe amplifier tube It may function also as a limiter. The tube It mayhave its input grid connected by a coupling condenser II to any desiredsource of FM signals. Specifically, and merely for the purpose ofillustration, it is assumed that the signal source is the usual'intermediate frequency (I. F.) amplifier network of a superheterodynereceiver. Those skilled in the art are sufficiently well acquainted withthe construction of FM receivers of the superheterodyne type so as tomake it unnecessary to describe in detail the networks there isdelivered to the 'detector network a car rier wave at I. F. which ispurely frequency modulated. The plate I2 of the limiter tube isconnected to a source of positive potential through a coil l3 providedwith an adjustable iron core H. The resistor I5 is arranged in serieswith the coil 13. The cathode of the limiter tube is connected to groundthrough a properly by-passed self-biasing resistor l6, while theinputgrid of the tube is grounded through the grid leak ll. To securelimiting action the voltages applied to the plate and screen grid oftube I 0, and the time constant of I ll1, are so chosen that the tubehas a horizontal characteristic relating signal input and signal outputvoltages above a predetermined value of signal input voltage,

The plate end of coil I 3 is connected by coupling condenser 20 to thejunction of discriminator coils 2i and 22, each of the coils 2i and 22being wound co-axially on an insulation form 22. Coil 2| is connected toground through the series condenser 24, whereas coil 22 is connected toground through the 1 series condenser 25. The diode 26 is connected inshunt across condenser 24, whereas the diode 21 has its anode connectedto the'junction of coil 22-and condenser 2I.-' The voltage.

cathode. of diode'2l is connected to ground through the resistorsections 28 and 23. It is pointed out that resistor 29 functions as thethe junction of the two resistors is connected by lead 30 to thejunction of coils 2| and 22.

Audio frequency voltage is taken off from the cathode endof resistor 28,and this voltage is passed through a filter network 3| prior toutilization in any desired type of audio frequency amplifier circuit. Ofcourse, the audio frequency voltage corresponds to the frequencydeviations imposed on the carrier at the transmitter, the amplitude ofthe audio frequency voltage corresponding to the frequency deviation ofthe carrier, whereas the-frequency of the audio voltage corresponds tothe rate of change-of the ire-,- quency deviation. Automatic frequencycontrol voltage (AFC) may be taken oil from the output end of filter 3|,9. further filter 32 being employed to eliminate the audio pulsationsinthe As known to those skilled in the art the AFC voltage may be usedto control a frequencyicontrol tube which is operatlvely associated withthe local, oscillator tank circuit in the manner of an electronicreactance. AFC network functions tov adjust the tank circuit frequencyin a sense such that the predeter- The.

vided at the opposite ends of the form 23 so as to provide bearings forthe threaded stems 42' and 44. Condensers 24 and 25' are chosen so as totune the respective series resonant circuits correctly with the ironplugs at positions near the middle of their effective range ofadjustment. That is, with the plug 40 adjusted to occupy a. positionabout half. way into coil 2|,

' the series resonant circuit 2l-24 is tuned to a frequency ii on oneside of the mean frequency quencies n and in. The mere fact that theserviceman need only manipulate knob SI for future adjustments provides:an important feature of this invention. In place of the ring" 50 theremay be used a thin powdered iron disk, as its effect would vary withangular setting. Also, if a slot becut in the form between the coils aniron wedge could be inserted in such slot for the same purpose.

- The impedance matching network between the limiter tube and thediscriminator has the further advantage that the inductance element I3acts as a direct current supply path for the plate circuit ofthe tube.Thecapacity element 2.0 acts as a direct current blocking condenser tokeep the high voltage of the plate supply away from the detectingsystem. The. values of inductance and capacity required for the networkIll-20 depend upon the impedance of the tuned circuit network, and thelatter in turndepends upon the series resistances of these circuits. Thecoil l3 and condenser 20 cooperate to provide a current step-uptransformer. This transformer In the above formulae R1 represents theplate resistance of the limiter, tube; R2 designates the mid-frequencyresistance of the circuit of Fig. 1';

while ,fm is the mid-frequency. The core 14 is adjusted to satisfy theabove equations. This is also true for condenser 20. The circuit l3-2lloperates by resonance to match the high impedance tube III to the lowresistance load of the detector circuit. Since the coils2l and 22 inFig..

5 are wound on the same form 23 there is mutual inductance between thecoils. A more involved analysis indicates that the behavior of thesystem is generally the same as described, and that the value of the Qof the whole coil assembly justed to tune the respective series resonantbranches to their predetermined. frequency as above described.

Thereafter, for alignment, only ring 50 need be adjusted as itadjustsboth series resonant frequencies simultaneously while leaving thedifference, or mean, frequency between them substantially unaltered.This is of considerable im-- should be in' the vicinity of one totwotimes the ratio W I Channel width The coil l 3 and condenser 20provide-a current step-up transformer. By properly choosing theconstants of l3-20 any given small load resistance can be transformed toa higher input resistance between input terminals. Ate the .same

time the load current is found to be greater than the input current. Thegreater the impedance change, the greater will be the current step-up.In Fig. 5 the input impedance is the high impedance of tube III, whilethe output impedance is the low resistance of thediscriminator-rectifier network. Hence, the current flow through theload of [3-20 is greater than the current flow through the high inputimpedance.

While I have indicated and described several systems for'carryingmyinvention into ieifect,

it willbe apparent to one skilled in the art that my invention is by nomeans limited to the particular organizations shown and described. but

tube It and the discriminator set forth in the appended claims.

that many modifications may be made without departing from the scope ofmy invention, as The generic term angular velocity-modulated carrierwave spaced from the center frequency of the carrier waves in oppositedirections, and an impedance matching network coupling the outputelectrodes of said tube to the junction of said pair of resonantcircuits and said matching network comprising an inductance in shuntacross the said output electrodes and a condenser in series with saidinductance.

2. In combination with a source of frequency I modulated carrier waves,a transmission tube having its input electrodes coupled to said source,a detection network comprising a pair of series resonant circuitsarranged in parallel, each of said resonant circuits being tuned to afrequency spaced from the center frequency of the carrier waves inopposite directions, and an impedance matching network coupling theoutput electrodes of said tube to the junction of said pair of resonantcircuits, said matching network comprising a current step-uptransformer. a

3. In combination, a pair of series resonant circuits arranged inparallel across a source of angular velocity-modulated carrier waves,said resonant circuits being tuned to frequencies differing from-amid-frequency value by the same .frequency amount and on opposite sidesthereof, a rectifier operatively associated with each resonant circuit,

means for combining the rectifier outputs in polarity Opposition saidsource of carrier waves consisting of an amplifier tube, and a currentstep-up transformer coupling said tube to the junction of said resonantcircuits.

4. In combination, a pair of series resonant circuits, each resonantcircuit consisting of a pair of reactance of opposite sign, a source ofangular velocity-modulated carrier waves, said resonant circuits beingoppositely mistuned to frequencies differing from a given frequencyvalue by the same frequency amount, said circults being connected tosaid source, a separate detector of the diode type connectedacrosssolelyv onereactance of each resonant circuit, said onereactance'of each resonant circuit consisting of solely a condenser, andmeans for combining the detector outputs in polarity opposition.

5. Incombination with a source of angularvelocity modulated carrierwaves, a transmission tube having its input electrodes coupled to saidsource, a rectification network comprising a pair of series resonantcircuits arranged in parallel, said resonant circuits being oppositelymistuned from the center frequency of the carrier waves by an equalamount, an impedance matching net work coupling the output electrodes ofsaid tube to the junction of said pair of resonant circuits, and saidmatching network comprising a current step-up transformer.

6. In a frequency responsive network adapted to respond linearly withrespect to frequency defirst series tuned circuit resonant to a firstfrequency connected between said terminals, a second series tunedcircuit resonant to a second frequency connected between said terminals,the ratio of inductive reactance to series resistance in each of saidcircuits being, at the mean of said frequencies, of the order of theratio of said mean to the difference of said frequencies whereby theimpedance between said terminals is substantially constant over therange between said frequencies, means for creating a substantiallyconstant voltage across said terminals throughout said range, said'lastmeans comprising a devicefor supplying constant current to saidterminals, a detector associated with each of said circuits. and meansfor combining the output voltages of said detectors in opposition.

' 7. In a frequency responsive network adapted to respond linearly withrespect to frequency departures from a fixed frequency over a range offrequencies, a pair of signal input terminals, a first series tunedcircuitresonant to a first frequency connected between said terminals, asecpartures from a fixed frequency over a range of frequencies, a pairof signal input terminals, a

ond series tuned circuit resonant to a second frequency connectedbetween said terminals, each of said tuned circuits including a pair ofreactances of opposite sign, the ratio of inductive reactance to seriesresistance in each of said circuits being, at the mean of saidfrequencies, of the order'of the ratio of said mean to the difference ofsaid frequencies whereby the impedance between said terminals issubstantially constant over the range'between said frequencies, meansfor creating a substantially constant voltage across said terminals.throughout said range, said last means comprising a device forsupplying constant current to said terminals, a separate detectorconnected across solely one reactance of each of said tuned circuits,and means for combining the output voltages of said detectors inopposition.

8. In a frequency responsive network adapted to respond linearly withrespect to frequency departures from a fixed frequency over a range offrequencies, a pair of input terminals, a source of frequency modulatedcarrier energy connected to said terminals, a first series tuned circuitresonant-to a first frequency connected between said terminals, a secondseries tuned circuit resonant tofa second frequency connected betweensaid terminals, each of said tuned circuits including a pair ofreactances of opposite sign, said two frequencies being equally andoppositely located relative to the carrier frequency, a separate de-.tector connected across solely one reactance of each of said circuits,means for combining the output voltages of said detectors in opposition,and a single means for adjusting the aforesaid two resonant frequenciessimultaneously while maintaining said equal and opposite frequencyrelation therebetween.

9. In a frequency responsive network adapted to respond linearly withrespect to frequency departures from a fixed frequency over a range offrequencies, a pair of input terminals, a source of frequency modulatedcarrier energy connected to said terminals, 8. first series tunedcircuit reso-- stant current of relatively small magnitude, a

rectification network comprising a pair of tuned circuits in parallelrelation, each circuit com-. prising an inductance in series with acapacity,-

said tuned circuits being series resonant to frequencies on oppositesides of the mean frequency of said waves, a separate rectifier deviceconnected across only the condenser of eachtuned circuit, an impedancematching network between the tube and said tuned circuits, said matchingnetwork comprising aycoil in shunt across the tube output electrodes,and a condenser in series relation between the junction of theinductances of said tuned circuits and said shunt coil.

11. In a frequency responsive network adapted to respond'linearly withrespect to frequency departures from a fixed frequency over a range offrequencies, a pair of signal input terminals, a first series tunedcircuit comprising a coil and a condenser resonant to a first frequencyconnected between said terminals, a, second series tuned circuitcomprising a second coil and a .second condenser resonant to-a secondfrequency connected between said terminals, the ratio of inductivereactance in series resistance in each of saidcircuits being, at themean of said frequencies, of the order of the ratio of said mean to thedifierence of said frequencies whereby the impedance between saidterminals is substantially constant over the range between saidfrequencies, a separate detector associated with the condenser of eachof said tuned circuits, means for combining the output voltages of saiddetectors in opposition, and a single mean constructed and arranged toaffect both said coils for adjusting the aforesaid two resonantfrequencies concurrently while maintaining said mean frequency at itsvalue.

WALTER VAN B. ROBERTS.

